SEOUL — North Korea is digging tunnels at a site where it has launched two nuclear tests, suggesting it is preparing a third, the South’s Yonhap news agency said on Sunday, a development which would trigger concern across the region.

Tensions rose on the divided peninsula when 46 sailors were killed in an attack in March on a South Korean naval vessel. North Korea, which has denied responsibility, shelled the southern island of Yeonpyeong in November, killing four people and sparking fears of possible all-out war.

The North was excavating several tunnels before picking the most suitable, Yonhap said, citing an unnamed South Korean government source.

“South Korean and U.S. intelligence authorities have spotted North Korea building several other underground mines at Punggye-ri where it had run two nuclear tests,” the unidentified source was quoted as saying.

“It is judged to be clear evidence of preparing for a third nuclear test.”